The invention relates to a hard magnetic object and method for adjusting a magnetic vector of a hard magnetic object according to the generic parts of claims 1 and 10.
For the varying mechanical, technical and medical applications the use of hard magnetic objects is known. Inter alia, hard magnetic objects are used for measuring devices and magnetic bearings. Magnetic bearings, especially for blood pumps, implanted as heart support pumps into the body of a human being, are in contrast to common bearings free of wear and gentle to the blood.
For some applications a more specific geometric alignment of the magnetic vector of a hard magnetic object is necessary, exceeding the common north-south alignment. Especially in bearings of blood pumps, an exact alignment and correction of the direction and of the position of the magnetic vector of the hard magnetic object is very important for ensuring the bearing clearance of the magnetic bearing.
FIG. 18 shows such an axial blood pump. The drive of the blood pump works according to the principal of an electronic commutated synchronous motor. The motor has a stator, consisting of a metal sheet packet 31, of windings 33 and iron flux return hoods 2, 2a, and a rotor 5 with a permanent magnetic core 32. The stator encloses a tubular hollow body 1, in which in axial direction a fluid, in the present case blood, is delivered. The rotor 5 is supported magnetically free of contact.
The magnetic support bearing consists of permanent magnets 42, 42a on the rotor end sides and permanent magnets 41, 41a on the end sides of the guiding devices 6 and 7. The guiding devices 6, 7 are mounted on the inner wall of the tubular hollow body 1.
To the magnetic support bearing further belong control coils 12, 12a. Sensor coils 43, 43a in the guiding devices 6, 7 and short circuit rings 80, 80a arranged opposed thereto, serve for measuring the actual rotor position.
The pairs of permanent magnets 41, 42; 41a, 42a are, respectively, polarised for attracting each other. Magnetically the pairs are arranged in series.
The control coils 12, 12a are connected electrically in series and are magnetically arranged in such a way, that a current weakens the magnetic field of the one pair of magnets and increases the magnetic field of the other pair. The magnetic flux return path is produced via the iron flux return hoods 2, 2a and the metal sheet packet 31 of the stator.
The axial position of the rotor 5 can be determined by means of the sensor coils 43, 43a. The sensor coils 43, 43a are loaded by a higher frequent voltage. By the axial movement of the rotor 5 a change of the inductively of the sensor coils 43, 43a is produced. By the arrangement of the sensor coils 43, 43a in a bridge connection a measuring signal for the axial position of the rotor 5 can be achieved.
For a bearing the bearing stiffness, the bearing clearance and the wear behaviour are generally characteristic. In a magnetic bearing the component guided in the bearing moves especially around or along an imagined magnetic axis without mechanical contact with other components of the device and independent of its mechanical geometry. During slow movements, depending on the application, a lower bearing stiffness and accuracy can be tolerated. Especially for fast rotational movements and/or large moving masses a high bearing stiffness within narrow tolerances is necessary because of the produced imbalance or the inertia of masses of the guided parts. In an axial blood pump used as an artificial heart support system for small dimensions high rotational speeds are necessary for the delivery capacity. To keep the stresses on the blood within justifiable limits in an optimised inner pump geometry, e.g. a maximal gap dimension between the rotor and the pump tube of 0.01 mm is to be maintained. Mechanical bearings (e.g. ball-bearings) would easily satisfy the mechanical requirements, but they destroy too much of the blood substance in the direct blood contact. If mechanical bearings for this application are sealingly inserted, the long term leak tightness, necessary for this application case, can not be ensured with the present State of the Art. Furthermore, at the transition between the shaft and the seal a blood damage is produced and an increased thrombosis danger exists at the boundaries of the seals. Pump rotors being free of wear and freely hovering by means of the magnetic forces, minimise these disadvantages. The bearing stiffness of the magnetic bearings of the rotor means, however, a limited bearing clearance, which cannot be undershot at a limited construction space and at hydrodynamic loadings necessary for the pump pressure. Additional bearing loadings caused by imbalances enlarge this bearing clearance. To minimise the imbalance, the magnetic bearing axis has to correspond as exactly as possible to the geometric bearing axis of the driven pump rotor. In the application case of the blood pump, for the limitation of the imbalance and for maintaining the clearance measurement, the angle deviations of the resultant magnetic vectors of the bearing magnets from the geometric rotational axis have to be below 0.3°. The common anisotropic highly coercive magnets, necessary for the capacity parameters of the magnetic bearing, have, however, measured averaged deviations of up to around 3° to the normal of the pole faces, which are oriented statistically as a bell curve distribution around the respective averaged value correspondingly to the base orientation of the starting material. Magnets traditionally made from the standard material in one piece, achieve only an immensely low yield of magnets, which have a resultant magnetic vector deviation of less than 0.3° to the pole normal.
The reason for this is, that the optimal or desired direction and size of the magnetic vector of a moulding opposes the statistically distribution of all the uncompensated spinning moments, which are responsible for the magnetic behaviour. Only in faultless single-crystals, single-range districts are present without a statistical distribution. Their application can, however, not be considered because of unsuitable material characteristics (e.g. a too low energy product) for the manufacture of magnetic bearings or other technically relevant devices. Also in materials with a distinct anisotropy a distinct statistical distribution of the uncompensated spinning moments is present with a fluctuation width, however, strongly limited. It is active macroscopically in statistical direction fluctuations of the resultant magnetic vector within a specific tolerance range.
In the most technical applications for the permanent magnets this fact plays an inferior role, as fluctuations of the magnetic vector, caused by the manufacture, around a desired zero position are tolerable.
In some applications, like, e.g. implantable blood pumps, the statistical direction fluctuations are, however, disadvantageous, as the application of permanent magnets with a magnetic vector, deviating from the desired direction, lead to an imbalance, which is too large, and therefore, to a bearing clearance, which is too large.
Therefore, it is necessary for such applications, to change or correct, respectively, the direction and position of the magnetic vector of a generally hard magnetic object in the open magnetic circuit. Such a change or correction, respectively, can be achieved in different ways.
A simple possibility is the application of an isotropic, hard magnetic material, which can be magnetised in the desired direction and strength. For such a method at the moment only hard magnetic materials are known, which cover in the maximal energy density only the lower range of the technical crest value. Materials with such a low energy density can, however, not find any application for magnetic bearings of the above described type, as the required bearing stiffnesses are not achieved.
Insofar as higher energy densities are necessary, the possibility exists, to realise the amplitude of the desired magnetic vector by means of selection of the magnetic material suitable for high energy densities, and the geometric form. The approximation to the desired direction of the magnetic vector to the geometry of the component can then be achieved when exactly knowing the position of the resulting magnetisation vector in the starting magnet by means of concerted “angle cutting”. Disadvantageous are an increased work expenditure and material consumption as well as hitting accuracy of the direction of the magnetic vector to be achieved only within a distinct deviation.
Furthermore, it is known, to realise a change of the magnetic vector by means of concerted demagnetisation or magnetisation, respectively, of partial areas or the totality of a hard magnetic object. This demagnetisation or magnetisation can be achieved by means of partial fields, asymmetrical fields, a changed field gradient or other methods. Disadvantages of this method are, that in general the energy content of the magnet is not used in the full extend. This is also valid, when a change of the magnetic vector is achieved by means of using the temperature dependency of the magnetic characteristics, i.e. by means of local asymmetrical warning or cooling, respectively. Furthermore, active influencing, e.g., by means of coupling with correspondingly formed and directed coils, which are variable in the correction possibilities by means of changed drive, are known. These necessitate, however, insertion space and additional energy.
The design of other hard magnetic objects and methods for the building up of magnetic arrangements are known from GB 777 315, CH 304 762, U.S. Pat. No. 4,777,464, U.S. Pat. No. 2,320,632, DE 21 06 227 A and DE 26 07 197 A1.
In U.S. Pat. No. 2,320,632 a method for connecting permanent magnetic and soft magnetic component(s) by means of casting on of magnetic material and forming as an integrally connected magnetic component, which takes up by a slot the thermal deformation during the cooling process is described. The permanent magnetical component is, in this case, arranged between the soft magnetic pole parts. Due to this, an influence on the direction of the magnetic field of the permanent magnetic component is not possible for the above named technical applications.
In U.S. Pat. No. 777,315 and CH 304762 a magnetic yoke as a connection between permanent magnetic and soft magnetic components is described. The yoke is part of a closed magnetic circuit, e.g. in an electrical measuring device. The permanent magnetical component is arranged between soft magnetic pole pieces. Because of this, an influencing of the direction of the magnetic field of the permanent magnetic component is not possible.
In DE 2106227 A as well as DE 2607197 A1 an air gap magnetic system is described. In this case, permanent magnetic parts are imbedded in soft magnetic parts in a magnetic circuit. An influencing of the direction of the magnetic field of the permanent magnetic part is not intended and would also not be realisable, as it would be destroyed by means of the abutting soft magnetic parts.
In U.S. Pat. No. 4,777,464 also a magnetic system with an air gap, having a closed magnetic circuit is described. To a soft magnetic outer yoke two opposed permanent magnetic parts are single-sidedly coupled with the same magnetisation to the inner sides, which are combined, respectively, from two magnetic materials. On the side of these permanent magnets facing each other, the working air gap is formed with a soft magnetic pole shoe formed according to the invention. Target of the arrangement is, to achieve an as far as possible constant field distribution in the working air gap. An influencing of the direction of the magnetic field of the permanent magnetic part is not intended. The used magnets should have the same direction. Each direction change would be destroyed at the soft magnetic yoke and at the pole shoe. The amplitude of the magnetic vector is in the classical sense achieved by means of a change of the geometric relationships and dimensions of the used different sorts of magnets.
In summary it can be pointed out, that with none of the known methods the direction of the magnetic field of a hard magnetic part can be influenced. The core of the above described methods is that, by means of the closed magnetic circuit the magnetic flux, possible with the provided magnets (or coils) is coupled to a maximum into the working field of this invention (air gap in U.S. Pat. No. 2,320,632, DE 2 106 227 and DE 2 607 197 and soft magnetic test objects in U.S. Pat. No. 777,315 as well as CH 304 762), or in U.S. Pat. No. 4,777,464 it is important, that in the air gap of the magnetic field at specific values, an as constant as possible distribution of the magnetic field is achieved.
Therefore, with the known methods an alignment of the magnetic field can not be achieved. Thus, the influence at a hard magnetic object for the direction of the magnetic field is directly cancelled out by the arrangement with a closed magnetic circuit, provided in the above named methods, by means of the soft magnetic parts abutting the hard magnetic object. Therefore, each previous change or adjustment of the direction of the magnetic field is cancelled. The soft magnetic parts, not working in the saturation, concentrate in the contact face towards the hard magnetic object the magnetic flux in dependency of the difference in permeability, however, independently of the direction. The magnetic flux extends within these parts in accordance with the difference gradient of the magnetic potential. The field exits perpendicular to the upper face of the position, at which the permeability jump to the surrounding or, to the neighbouring part takes place, the soft magnetic parts. The outer path of the field lines depends then on the provided outer magnetic field conditions. In the above described methods the field lines extend mainly within the predetermined magnetic circuits. With these arrangements an-influencing of the magnetic vector is not possible.